those of any comet which has been previously observed: if so (especially if the parabolic elements indicate a path through the heavens which is evidently not the path being followed by the comet), calculations for an elliptic orbit are undertaken and a period (usually stated in years and decimals of a year) is deduced. To compute an elliptic orbit for a comet or planet will take even an experienced calculator many hours of very hard work. An approximation may, however, be obtained by a graphical process.[1]
It will be desirable to give the reader a little further insight into the nature of cometary orbits without going more deeply than can be helped into mathematical matters.[2] The orbits of all comets, planets, and binary stars are in form sections of cones, whose size, exact form, and position in space are defined by numerical quantities technically termed "elements", which for brevity's sake are usually designated by symbols as follows:—
T = Moment of the body's Perihelion Passage or nearest approach to the Sun,[3] expressed in Greenwich, Berlin, or Paris time according to the nationality of the computer, Americans adopting Greenwich time.
- ↑ Such as that framed by Mr. F. C. Penrose, which is presented in my Handbook of Astronomy, 4th ed., vol. i, p. 491 et seq.
- ↑ Perhaps the best way of realising clearly the nature of a comet's orbit, and the difference between one orbit and another, is by making cardboard models. For instructions how to do this see an article by a distinguished American astronomer, Professor Harkness, in the Sidereal Messenger, vol. vi, p. 329. Dec. 1887. I have epitomised some portions of that article in the next following statement of the elements of an orbit.
- ↑ In the case of a binary star, of the nearest approach of the companion star to the principal star; in such case the point of nearest approach is called the peri-astron instead of the peri-helion.
